Análisis de la Demanda

PAH contaminants adhere to soils when they are released into the environment. Desorption of a PAH contaminant from the soil is essential for a successful remediation process. Three tests were carried out to investigate sorption and desorption kinetics for phenanthrene as a representative PAH compound and kaolinite as a soil matrix. The data from the batch sorption test was fitted by the Freundlich and Langmuir isotherms where the Freundlich isotherm was found to best represent the results. Desorption test results, after the batch sorption tests, showed that not all the sorbed phenanthrene by the soil matrix can be desorbed. The highest desorption percentage reported in the batch tests is 81% and the lowest is 76%. Desorption of phenanthrene by electroosmotic flow in all the tests was found to be significantly higher than desorption caused by hydraulic flow. For example, in the soil specimen with an initial phenanthrene concentration of 4000 mg/kg, the phenanthrene removed after one pore volume of electroosmotic flow was about four times the amount removed after one pore volume of hydraulic flow. Phenanthrene desorption by hydraulic flow increased approximately linearly with the number of pore volumes in soil specimens with concentrations of 500, 1000, and 2000 mg phenanthrene/kg of soil, whereas desorption by electroosmotic flow was approximately linear for phenanthrene concentrations of 500 and 1000 mg/kg of soil.

Typically, the Darcy’s velocity for ground water in kaolinite clay is around 2x10-9 m/s and there is no control over the direction of the flow of ground water. Electrokinetics can be used to generate electroosmotic flow with a velocity higher than ground water flow. Moreover, the direction of electroosmotic flow can be controlled by the orientation of electrodes. The results of this study showed that desorption of phenanthrene can be promoted by the use of electroosmotic flow. The removal of the phenanthrene from contaminated soil using electroosmotic flow was three to four times higher than in soil samples treated with hydraulic flow desorption. Moreover, the power required in the hydraulic test was found to be three orders of magnitude higher than the power requirement in the electrokinetic test.

3.9

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CHAPTER 4

A NOVEL TECHNIQUE FOR pH STABILIZATION FOR

ELECTROKINETIC BIOREMEDIATION

4.1 INTRODUCTION

Contamination of soil and groundwater by petroleum hydrocarbons occurs in various operations including exploration and production of oil, transportation of crude and refined oil, and improper management of refinery waste. Petroleum hydrocarbons (PHCs) exist in the subsurface as separate phase liquids immiscible with both water and air, referred to as Non-Aqueous Phase Liquids (NAPLs). Very low concentrations of these compounds can threaten human health and the environment. Over the years, many remediation methods have been used with various degrees of success to mitigate petroleum hydrocarbon pollution. Among these methods, recent studies have investigated an innovative hybrid technique that joins electrokinetics and bioremediation. The aim of this hybrid approach is to accelerate the natural biodegradation of contaminants by increasing the opportunities for interaction between microorganisms and contaminants and activating the existing microbial community in the subsurface by delivering nutrients required to promote microbial growth (Acar et al. 1997; Budhu et al. 1997). In a bioremediation process, there is an optimum pH range at which the capability of specific microorganisms to degrade a particular contaminant is maximized. Most bacteria can live in a pH range between 6 and 8; however, special kind of bacteria can tolerate extreme pH values (<2 or >10). Although bacteria can adapt the cytoplasm pH to the surrounding environment by controlling the exchange of hydrogen ions (internal proton concentration) through the cell wall, the abrupt change in pH gradient across bacterial membrane has an adverse effect on bacterial growth and metabolism (Cotter and Hill 2003; Padan et al. 2005; Krulwich et al. 2011). In electrokinetic remediation, the electric field incites three transport mechanisms, namely: electroosmosis, electromigration, and electrophoresis along with electrolysis reaction at the electrodes.

Electroosmosis is the movement of liquid in soil pores relative to a stationary charged soil particle under an applied electrical field. Electromigration is the transport of ions in the pores fluid towards the oppositely charged electrode whereas electrophoresis is the movement of charged colloids under an applied electrical field (Acar and Alshawabkeh 1993). Electrolysis reactions occur at the electrodes in an electrokinetic process and result in oxidation-reduction reactions (Alshawabkeh 2009). Oxidation takes place at the anode, which generates hydrogen ions (acid front) and liberates oxygen gas. On the other hand, reduction occurs at the cathode, which produces hydroxyl ions (base front) and disperses hydrogen gas (Acar and Alshawabkeh 1993).

Oxidation reaction at the anode:

2𝐻₂𝑂 − 4𝑒− _{→ 𝑂}

2(𝑔) + 4𝐻+(𝑎𝑞) (4.1)

Reduction reaction at the cathode:

4𝐻2𝑂 + 4𝑒− → 2𝐻2(𝑔) + 4𝑂𝐻−(𝑎𝑞) (4.2)

Where: O2 (g) is oxygen in the gaseous phase, H+(aq) is hydrogen in the aqueous phase,

H2 (g) is hydrogen in the gaseous phase, and OH-(aq) is hydroxyl ions in the aqueous phase.

The acid front (i.e. H+) moves towards the cathode by electromigration, electroosmotic flow, and diffusion and lowers the pH of the soil along its path. The hydroxide ions that form the base front travel towards the anode by electromigration and diffusion and elevate the pH of the soil in the vicinity of the cathode (Acar and Alshawabkeh 1993). The drastic change in soil pH (acidic near the anode and alkaline near the cathode) plays a very important role in the outcome of the mitigation of petroleum hydrocarbons by electrokinetic bioremediation (Alshawabkeh 2009). The development of pH gradient in the soil by electrolysis reactions of water is detrimental to the existence of bacteria and can subsequently decrease the effectiveness of electrokinetic bioremediation. For example,

Kim et al. (2010) observed that changes in soil pH during electrokinetics had reduced microbial cell number and microbial diversity.

Much research has investigated means to neutralize soil pH during electrokinetic remediation. This includes the use of ion selective membranes (Hansen et al. 1997); addition of chemical conditioning agents such as ethylenediaminetetraacetic (EDTA) (Reed et al. 1995; Wong et al. 1997), acetic acid and nitric acid (Denisov et al. 1996); continuous changing, removal and circulation of the solution in the electrode compartments (Niqui-Arroyo et al. 2006); polarity reversal (Luo et al. 2005; Pazos et al. 2006); and configurations of electrodes and electrical fields (e.g. unidirectional, bidirectional and rotational operations) (Luo et al. 2006). The practice of replacing the electrolyte solution results in solution pollution that requires treatment before release into the environment. The addition of a chemical conditioning agent is not favorable because it generates undesirable by-products and adds additional expense. Furthermore, the use of acids to control the pH level can acidify the contaminated soil which makes it very difficult, if not impossible, to restore soil to its previous condition (Hassan and Mohamedelhassan 2014).

Although polarity exchange may result in a neutral soil pH during electrokinetic bioremediation, the technique requires continuous pH monitoring which can be challenging and increase the overall cost of the process. Polarity exchange also results in bidirectional (i.e. the direction is reversed with polarity change) movement of the pore fluid, nutrients, and bacteria (Luo et al. 2005). The technique can increase bioavailability provided the time before polarity exchange is sufficient to cause the bidirectional movement. However, a long duration before polarity exchange increases the pH at the cathode and decreases the pH at the anode which is not favorable to the bioremediation process. For example, Kim et al. (2005a) stated that “it is not feasible to control the pH using the polarity exchange technique”. In a study by Pazos et al. (2006), it was found that the low pH environment induced at specific compartment when the electrode is used as an anode had not increased when the specific electrode used as a cathode. This provides

evidence that the polarity exchange technique can not be used to keep the pH at the electrode compartment to remain unchanged. Research by Kim and Han (2003) implemented the circulation of electrolyte solution in order to keep the pH at the electrode compartments neutral, an approach that can be effective and suitable for electrokinetic bioremediation treatment. However, the circulation of electrolyte solution (anolyte and catholyte) in a field application can be difficult and costly due to the need for continuous pumping during the treatment.

As the goal of bioremediation is to use microorganisms to degrade pollutants to less harmful products, the success of the technique would depend on the growth and reproduction of microorganisms. Often, nutrients are necessary to stimulate microbial growth and metabolism. Electrokinetics transports ions through the soil and allows for the control of the direction and magnitude of movement. Therefore, when combined with bioremediation, electrokinetics can deliver nutrients to indigenous bacteria in the soil and increase mixing between bacteria and contaminants. Many studies have investigated the delivery of nutrients using electrokinetics. For example, Schmidt et al. (2007) demonstrated the feasibility of transporting two common microorganism nutrients (nitrate and ammonium) by electrokinetics in a tropical clayey soil. The results, however, showed nonuniform nutrients distribution with a higher amount of nitrate transported to the soil in the vicinity of the anode compared with the ammonium transported to the soil near the cathode. Xu et al. (2010) observed that in electrokinetics bioremediation tests using polarity exchange to control pH, a relatively even distribution of nutrients in the soil was achieved compared with tests that used one direction electric fields. The results from electrokinetic bioremediations studies have shown that electrokinetics is successful in delivering nutrients to indigenous bacteria. However, excessive amounts of nutrients in soil can exploit microbial growth and increase the population of microorganisms and consequently lead to clogging of soil pores and, eventually, fouling (Kim and Han 2003). Therefore, it is important to study and carefully plan for the addition of nutrients. The implementation of the aforementioned approaches in field applications increases the complexity of the process and needs either the addition of chemical compounds or

provision of extra personnel supervision/intervention or both. Thus, the overall cost of the remediation process increases regardless of the improvement in the effectives of the process.

In electrokinetic bioremediation applications, the control of soil pH is crucial for a successful treatment. The methods presented to date in the literature show great advancements in the effort to control pH during electrokinetic applications. However, more research should be conducted to further improve existing methods and to develop new and innovative techniques to control the pH during the electrokinetic bioremediation applications. To date, no innovative low cost pH control techniques have yet been investigated.

The present study proposes a novel approach, anode-cathode-compartment (ACC), to stabilize pH and distribute nutrients in soil in order to enhance electrokinetic bioremediation of soil contaminated with biodegradable compounds. The goal of the study is to demonstrate the effectiveness of the novel ACC approach in keeping soil pH and water content relatively unchanged during an electrokinetic process. The distribution of nutrients in the soil by electrokinetics employing ACC configuration was examined and compared to distribution using conventional anode-cathode (CAC) configuration. The new proposed ACC technique overcomes the shortcomings of other pH stabilization techniques by stabilizing the pH without the need for pumping or amendments while maintaining the resultant electroosmotic and electromigration movement in one direction.